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4chord MIDI

4chord MIDI - the USB MIDI keyboard to play every major hit pop song with four little buttons.

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4chord MIDI - the USB MIDI keyboard dedicated to play all the four chord songs, from Adele via Green Day and Red Hot Chilli Peppers to U2 and Weezer. Thanks to MIDI, you can be any instrument - and all of them at once. Yay!

Yes, there's an undeniably vast amount of famous pop and rock songs out there using a chord progression built around the Tonic (I), Dominant (V), Submediant (vi) and Subdominant (IV). You won't even need the submediant to cover all of blues, country and punk music along the way. So why bother learning the full spectrum of an instrument (or, for that matter, learning any of the music theory terms I was just throwing around) and not just build one focusing on the essence of it. This is exactly what I did with 4chord MIDI.

What once started as a tongue-in-cheek idea to mock the simplicity of mainstream music has since grown into a full-blown project. Dabbling with songwriting myself taught me a thing or two about the infamous I-V-vi-IV progression over time. For one thing, it's catchy and it simply works, and I caught myself often enough accidentally ending up with it, despite all the mocking. But well, there's anyway a lot more to a song than the chord progression alone. After all, some exceptions (and cynicism) aside, all those famous "same" songs are far from being identical.

The Project

As stated in the description, 4chord MIDI is a USB MIDI keyboard, and as such, it works and behaves like any other MIDI instrument. The main difference is that the playback options are limited to four buttons, to play those mentioned four chords.

To make things more interesting and varied, any key can be selected as base for the chord progression, and the playback itself can be either a simple chord triad, an arpeggio, or a combination of both in a user defined tempo. In the first version, this is rather limited to 4/4 metre and quarter notes with very simple arpeggio patterns, but this is going to change in the future firmware development.

The Hardware

4chord MIDI is USB powered and mainly built around an ATmega328P, a good old Nokia 5110 display, and seven buttons - four to play the chords, and three to navigate the user interface.

Initially designed in credit card form factor (as you can see in the introduction video above), revision B was re-designed to get a more musically appropriate shape:


One recent design experiment took the music shaped theme then one step further:


For now I keep the focus on the piano shape, but I really like the guitar. The routing on the "amp" is pretty horrible though and nothing I'm comfortable to release at this point - I mainly wanted the guitar shape, so I just botched a quick and dirty amp design as proof of concept together. But that's going to change some time in the future.

Hardware Licenses

All 4chord MIDI hardware documents, such as the schematic and PCB design are released under the terms of the CERN Open Hardware License (OHL) version 1.2.

The Firmware

Since there's no hardware USB controller in the ATmega328P, one central part of the firmware is the Objective Development V-USB library, set up to implement the USB MIDI device class specification reference design (see Appendix B). The rest is some custom designed code written in C, to be compiled with avr-gcc and flashed via avrdude - nicely wrapped into a Makefile.

In the big picture, the firmware takes care of three main things:

  1. everything USB, handled by the V-USB library
  2. user interface settings and displaying it on the LCD
  3. chord playback based on the user interface settings

There's an additional UART interface for debug output and user input - you can basically emulate the button presses through a serial console with it.

Firmware Licenses

Both the V-USB library and the 4chord MIDI firmware are released under the terms of the GNU General Public License (GPL) version 2.

The Mission

Shits, giggles, and dominating the wedding band business.

Apart from that, well 4chord MIDI is neither going to revolutionize how we create music, nor will it change the world in any significant way. It will, however, provide everyone interested actively in music - be it song writing or just covering songs - a simple and convenient way to do so, packed in a nifty little package....

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  • 1 × ATmega328P TQFP32 Microcontroller
  • 1 × Nokia 5110 LCD Clip-in version taken from the usual breakout boards
  • 1 × USB Mini B SMD connector Connector
  • 7 × SMD tacticle switch 4 white and 3 black to achieve some piano-ish look, but doesn't really matter
  • 1 × 6pin IDC connector Serial programming connector

View all 19 components

  • FEED ME FIRMWARE!

    Sven Gregori10/15/2017 at 14:46 0 comments

    When I put together the firmware roadmap, it became clear to me that having an easy and convenient way to update the board with any future versions will be one of the most important features. With such a feature, I don't have to worry too much about missing features and can go ahead and offer 4chord MIDI to the world. Sure, there's the ISP connector, so updating the firmware has always been possible, but like I wrote in the previous log, I want that everyone can fully enjoy 4chord MIDI, regardless of their programming or electronics background.

    Well, today I finalized the first proof of concept implementation of a bootloader that will be able to do just that: updating the firmware.

    Thanks to the ATmega328's built-in bootloader support, it was surprisingly easy (almost disappointing) to have separated application and bootloader code. But of course, the main focus was the self programming and transferring the firmware data, so good to not have to worry about the linker along the way.

    Like I said, this is a first proof of concept, but it has full support to

    • receive new firmware data one memory page at a time - a Python script takes care of that
    • write the firmware data chunks to the flash
    • read the freshly written data back and send it back to the Python script which is then either moving on to the next memory page or sending the last one again, until the data matches
    • displaying some progress information (okay that one needs still some work)

    Here's a little video showing the firmware upgrade in action:

    Pressing the "Select" menu button while powering up the board will enter the bootloader mode. It has a simple UART interface to reset / handoff to application, but pressing the "I" chord and "IV" chord button at the same time will also reset the board.

    Next step is to fully integrate it into the Makefile and push some code to the Github repo.
    Coming soon. Development branch for bootloader is now here: https://github.com/sgreg/4chord-midi/tree/feature/bootloader

  • Firmware roadmap

    Sven Gregori09/04/2017 at 14:56 0 comments

    Every time an idea pops into my mind, I tell myself to finally get a notebook and start collecting them in there, yet every time I end up scribbling them on whatever random piece of paper I find lying around me that doesn't have a shopping list or any other thought on it yet. Well, now I finally made a scrap book of shreds of papers, old envelopes and Post-it notes related to 4chord MIDI, and turned all plans and ideas about the firmware future into Github milestones.

    I'll add issues for each individual feature to the Github repository as the milestones progress, but to give everyone else an idea what will come in the near and distant future, I thought this is a good place to tell about them.

    If you check the project's history, you'll see that the current state was mainly developed back in 2015, and at this point, moving from a credit card shaped PCB to a piano shaped one is the only real change that happened since. For me, this sounds like a good moment to increase the firmware's major version number and go for a 2.x path for any new feature. Keeping in mind that I want to take this project to @Tindie as soon as possible, yet have a long list of features I'd love to add, it made sense to already plan for different releases to separate need-to-have, want-to-have and nice-to-have features. These features are now planned to be implemented in firmware version 2.0, 2.1 and 2.2 respectively. Obviously that separation was done at my own discretion and may leave some definition a bit blurry.

    Version 2.0

    The current firmware has on the one hand everything you need - chords and arpeggios in every key at "any" tempo - but is on the other hand too limited to have full-blown fun with it. For instance, the arpeggio playback is limited to quarter notes in 4/4 metre, and you need to know all keys by heart if you're interested to know what the submediant (vi chord) of, say, F# would be. A nice rhythm using eighth notes in either 3/4 or 4/4 (and more later on) metre as a playback mode, displaying the I-V-vi-IV chords for each key and highlighting the one that's currently played, now that's a whole different story. So is the ability to update future firmware versions without the need of any external components or even any knowledge of programming a microcontroller.

    I've been looking for a reason to play around with the ATmega's bootloader memory area ever since I learned it existed. And while I should probably focus on other things in this phase, I'd still prefer being able to hand out 4chord MIDI hardware to an average person without any previous background in programming or electronics, so they can enjoy my creation.

    All this being said, the plans for the version 2.0 firmware release are:

    • changing quarter note based playback to eighth note playback
    • adjust the playback modes to make use of eighth notes, and add some new playback modes
    • adding metre choices for 4/4 and 3/4, and possibly more to come later
    • displaying the tonic (I), dominant (V), submediant (vi) and subdominant (IV) of the currently selected key
    • highlighting currently played chord on button press
    • redesigning the UI to include all these new features
    • implementing a bootloader to update future firmware releases without any need for programmers or development environments
    • adding long button press to the menu keys
    • cleanup the V-USB MIDI descriptors

    Version 2.1

    In theory, those 2.0 changes should add a bit more flexibility and comfort, but things are still a bit generic. If I've learned anything in my life, it's that people most certainly have different preferences over very much everything, Being generic will therefore cater equally bad to most, which can be considered fair in a way, but customizing to your own taste is probably the better choice. The next firmware iteration is therefore focusing on user configurable settings.

    In the big picture, this means there will be a new text based configuration menu introduced, separated from the main user interface. With long button press implemented in...

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  • 4chord MIDI Piano PCB is ready

    Sven Gregori08/19/2017 at 15:02 0 comments

    The revised PCBs arrived earlier this week and later the same night, I was holding another fully assembled 4chord MIDI piano prototype in my hand. Moment of truth has come: will this finally be the revision I'm happy enough to share it with the world and start offering to strangers?

    Initial situation looked good, I attached an USBasp programmer and checked if avrdude could recognize and communicate with the controller. No smoke, no short circuit, ATmega328p recognized - all good. Flashed the firmware, remembered to burn the fuses to use the external oscillator right away this time, and excitedly watched the syslog output as I plugged in the USB cable for the first time.

    New USB device found, idVendor=1209, idProduct=deaf

    What beautiful words.

    The LCD back light went on and the 4chord MIDI logo appeared on the LCD itself. It was one of those Disney moments you just want to burst into song. So I started up fluidsynth.

    Long story short, latest revised Piano shaped 4chord MIDI keyboard PCB seems to work all well and nicely, the toppling problem is solved, and the back light works.

    Time for some pictures and show the (admittedly minor) differences between the two PCB iterations.

    On the left is the initial version, on the right the revised one. Ignore the buttons on the right, I'm currently on the (seemingly futile) mission to find the perfect tactile switch.

    Main change is the fourth rubber foot on the top, slightly lowered silk screen on the buttons and the voltage selector for the UART connector.

    The bottom side, has mostly re-arranging some components and re-routing the tracks accordingly and some silk screen related changes, and of course fixing the back light transistor arrangement (which is also a MOSFET now).

    Again, left side the initial PCB, right side the revised, latest version.

    So, yeah, I'm happy enough with all of it, so this will be the version that is going to be shared on the OSH Park website. The KiCad and Gerber files have been merged to the Github master branch and tagged as release.

    Next steps are finalizing my preparations for a first small batch that will eventually make it to Tindie. But as I mentioned, I'm currently on some quest for buttons. Some year ago or so I ended with a random eBay order of tactile switches (the ones used for the chords), which are like no other switch I ever met. They are simply smooth and silent with a nice pressure point and feeling to it (Omron B3S series as it turned out). Would be great to find the same type of switch for the menu buttons - preferably in black (ebony would be perfect since those Omron ones are actually ivory), but so far no luck.

    So I'll keep up the quest for a little while longer and little by little set up everything else - including some new firmware features.

  • Revised Revision B iteration finalization

    Sven Gregori08/09/2017 at 23:23 0 comments

    Yes.

    Finished all items on my TODO list, and threw in some extra changes to - hopefully - end up with a slightly less amateurish PCB design. I'm still working on finding some better buttons than the cheap ones I have lying around for the menu navigation. They simply need a bit too much force and I'm just not happy with their feel. Especially in comparison with the chord buttons that are just so damn smooth. But knowing what to look for now, will make this a smaller issue.

    Apart from that, latest changes are in the Github repo in the feature/hardware-revision-b branch (once I get the latest iteration and am happy with it, it will be merged in the master branch), and I just sent the design to OSH Park (which will have the PCB as shared project the same time the feature branch gets merged to master).

  • Unique USB identifier

    Sven Gregori08/03/2017 at 18:32 3 comments

    Thanks to the folks at http://pid.codes/, 4chord MIDI has now its own unique USB vendor/product ID pair! In practice this means that one could add a udev rule specific to 4chord MIDI, and for example automatically execute a shell script whenever a device gets plugged in, which could then start fluidsynth and automatically connect it via aconnect. Or start jackd and a2jmidid to get Ardour ready for MIDI recording.

    Well, I guess those two example udev rules just made it on my TODO list..

  • Hardware Revision B

    Sven Gregori07/28/2017 at 18:43 0 comments

    Earlier this week, the fully re-designed revision B PCBs have arrived from @oshpark, and they are even more beautiful than I imagined. You simply cannot emulate this shininess in KiCad's 3D Viewer ..or capture it on a picture, but here is one nonetheless:

    The main change is obviously the PCB shape. I honestly don't know why I thought a MIDI keyboard had to be credit card sized in the first place, it's not like you'd carry it around in your wallet along with a battery powered computer to actually use it. So I figured, why not give it a more musicy look.

    Also there were a few issues I didn't like in my initial design and wanted to change regarding the crystal oscillator (too high frequency for 3.3V supply voltage and a form factor which wasn't that easy to source) and the LCD (also sourcing situation, couldn't fit the back light, and they kinda kept on dying on me). Also the horizontal button arrangement for the chords playback didn't feel very ergonomic or natural.

    So with the new boards designed, manufactured, and right in front of me, last night was the time to turn one of them into a functional prototype:

    I emphasize prototype here, because I realized a few things along the way:

    • There's really no need to have zero Ohm links in the power supply paths, this is a USB powered device. While analyzing and optimizing current consumption is fun and interesting, there simply is no reason to be able to do that in the final design.
    • Real pianos may be nicely balanced with three legs, but they don't have a huge USB connector in their upper end either. Pressing the "<" button will topple the board with the current mechanical arrangements, so that will need some adjustments.
    • On very first try, the LCD back light didn't turn on, and I haven't had time to look into it yet, so there is a possible issue that needs some re-work. Maybe I should just stop using BJTs and finally get more familiar with FETs.
      • Edit: turns out I used the wrong generic NPN transistor in the schematic, and base and emitter ended up swapped. Removed the SOT-23 transistor, bent some legs on a TO-92 to replace it, and back light turned on just fine.
    • Santa is coming early this year and he's so gonna bring some hot air soldering equipment.

    So there will be another iteration in the near future taking care of all these things, and once I'm satisfied with the results, the PCB will be available from OSH Park's shared project section, and some assembled ones will make their way to Tindie. And in the mean time, I have a whole list of new feature ideas for the firmware to work on, so I won't get bored.

    (Side note on Tindie, if you'd be interested to get a 4chord MIDI keyboard, drop me a message so I could get a rough picture)

View all 6 project logs

  • 1
    Simple MIDI playback with fluidsynth

    Install fluidsynth and sound fonts for it (YMMV on other Linux distributions)

    sudo apt-get install fluidsynth fluidsynth-soundfont-gm 

    Start fluidsynth

    fluidsynth -a alsa -m alsa_seq -g 1.0 -s /usr/share/sounds/sf2/FluidR3_GM.sf2 

    Check on a separate terminal for client:port numbers via aconnect and look for "4chord MIDI" input and "FLUID Synth" output

    aconnect -i
    aconnect -o
    aconnect 20:0 129:0

    Start playing music, that's it.


    For more details, check this video:

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Discussions

untimony wrote 09/29/2017 at 04:23 point

this is cool!

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oshpark wrote 09/29/2017 at 04:04 point

Love the guitar board! 

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davedarko wrote 08/18/2017 at 21:01 point

Okay, just saw the video and I really like it :) Nice work!

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Sven Gregori wrote 08/19/2017 at 11:11 point

Cheers :) will do some more at some point, using the new hardware, now that it seems to be ready enough.

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danjovic wrote 07/29/2017 at 14:59 point

I just have seen this project featured on Hackaday! It's awesome!!

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Sven Gregori wrote 08/19/2017 at 11:03 point

Thanks :) was quite a nice surprise when I saw it there myself.

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Ondřej Petrlík wrote 07/28/2017 at 14:18 point

Wow, interesting idea! Good job, thinking about building own copy asap.

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Sven Gregori wrote 07/28/2017 at 19:07 point

Thanks! I just wrote an update on the new hardware I designed and mentioned it will eventually be available on OSH Park and Tindie (so far the plan).

If you want to take care of everything yourself, the Revision A hardware has the KiCad project and Gerber files on Github, Revision B has only the KiCad project so far and is in its own branch (I could add Gerbers though - but as I wrote in the update, some minor changes are still coming).

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oshpark wrote 07/25/2017 at 18:26 point

Great project!

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Sven Gregori wrote 07/28/2017 at 19:01 point

Great service ;)

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